The place and date of the first publication of the papers here collected are mentioned in each case at the bottom of the first page of each chapter. I wish to thank the editors of the various periodicals for giving me permission to include these papers in the present book.
I have been helped in various ways with the revision of the text, the reading of the proofs, and the preparation of the index, by Richard Gombrich, Lan Freed and Dr. Julius Freed, J. W. N. Watkins, Dr. William W. Bartley, Dr. Ian Jarvie, Bryan Magee, and A. E. Musgrave. I am greatly indebted to all of them for their help. My greatest indebtedness is to my wife. She has worked on the book even harder than I, and her acute criticism has led to innumerable improvements.
K. R. P.